Protein kinases (PKs) are a large class of enzymes that catalyse the transfer of the γ-phosphate from adenosine triphosphate (ATP) to the hydroxyl group on the side-chain of a serine, threonine or tyrosine residue in a substrate protein or peptide. PKs are intimately involved in the control of various important cell functions, including signal transduction, differentiation, proliferation and apoptosis. Due to their role in critical cellular functions, PKs are logical targets for now therapeutics. Inhibitors of PKs have a variety of potential therapeutic applications including in the treatment of cancer, inflammatory conditions, autoimmune disorders, neurological disorders, diabetes and cardiac diseases.
Individual PKs within the PK superfamily can be classified as either serine/threonine-specific PKs or tyrosine-specific PKs according to the residue in the substrate protein/peptide that the PK phosphorylates. Both serine/threonine and tyrosine PKs have been the target of therapeutic strategies, particularly with regard to the development of new anti-cancer therapeutics, and, as a result, a number of PK inhibitors are currently in clinical development (see reviews by Goekjian & Jirousek, Curr. Med. Chem. (1999) 6:877-903; Goekjian & Jirousek, Expert Opin. Investig. Drugs (2001) 10:2117-2140; Sachsenmaier, Onkologie (2001) 24:346-355; Swannie & Kaye, Curr. Oncology Rep. (2002) 4:37-46, and Dancey & Sausville, Nat Rev Drug Discov. (2003) 2:296-313). Most of the PK inhibitors in clinical development are natural product analogues (for example, staurosporin or bryostatin analogues), small molecule inhibitors, monoclonal antibodies or antisense oligonucleotides.
A number of peptide-based PK inhibitors have also been described. For example, U.S. Pat. No. 4,582,821 describes peptide and amino acid halomethyl ketones that inhibit the activity of cAMP-independent serine or tyrosine PKs. U.S. Pat. No. 6,090,929 describes protein-binding fragments of gravin that can be used as inhibitors of cAMP-dependent protein kinase (protein kinase A or PKA) or protein kinase C (PKC). International Patent Application No. PCT/US99/22106 (WO 00/18895) and U.S. Patent Application No. 2002/0160478 describe peptide inhibitors of PKs that are derived from the sequence of the αD region of a PK. U.S. Patent Application Nos. 2002/0137141 and 2002/0115173 describe peptide inhibitors of PKs that are derived from the sequence of the A region of a PK. U.S. Patent Application No. 2002/0049301 and U.S. Pat. No. 6,174,993 describe peptide inhibitors of serine/threonine PKs that are derived from the sequence of the HJ loop of a serine/threonine PK, and International Patent Application No. PCT/US00/32852 (WO 01/42280) describes peptide inhibitors of PKs that are derived from the sequence of the B4-5 region of a PK. International Patent Application No. PCT/EP93/00816 (WO 93/20101) describes peptide inhibitors that specifically target PKC isotype zeta.
Peptide inhibitors that spontaneously form intermolecular disulphide bridges with the active site region of PKC isoforms have also been described (Ward et al., (1995) J. Biol. Chem. 270:8056-8060; Ward et al., Arch. Biochem. Biophys. (1999) 365:248-253). The peptides have the sequences RKRCLRRL (SEQ ID NO:27) and RRRCLRRL (SEQ ID NO:28) and, due to the nature of their interaction with the PKC enzyme, their inhibition of these enzymes is sensitive to reducing agents, such as dithiothreitol.
Various methods of identifying or optimising PK inhibitors have also been described. U.S. Patent Application No. 2003/0143656, for example, describes a method of identifying compounds that modulate PK activity based on the identification of a small hydrophobic pocket on the small lobe of PKA and the observation that compounds that interact with this hydrophobic pocket can modulate the activity and/or stability of a PK. International Patent Application No. PCT/US00/00803 (WO 00/42213) describes a modular strategy for developing non-peptidic PK inhibitors, which comprise a first module (M1) having functional groups that bind catalytic residues of the PK and a second module (M2) that provides a non-peptide scaffold.
The strategy involves a first step to identify suitable M1 modules that comprises binding candidate modules to a pentapeptide scaffold. Subsequent steps replace the pentapeptide with a non-peptide scaffold.
Other PK inhibitors have been described that include a peptide linked to ATP, or a derivative thereof. Medzihradszky et al. (J. Am. Chem. Soc. (1994) 116:9413-9419) describe potential cAMP-dependent protein kinase inhibitors that comprise ATP, ADP or adenosine tetraphosphate linked to the oxygen atom in the side-chain of the serine residue in kemptide, a known peptide substrate for cAMP-dependent protein kinase having the sequence LRRASLG (SEQ ID NO:29). U.S. Patent Application No. 2002/0031820 and Parang et al. (Nature Struc. Biol. (2001) 8:37-41) describe a similar type of inhibitor for inhibition of the insulin receptor tyrosine kinase or PKA that comprises ATPγS linked to a peptide substrate analogue by a two-carbon linker. U.S. Patent Application No. 2005/0026840 describes inhibitors of PKB that comprise small molecules having affinity for the ATP binding site of PKB, specifically small molecules comprising an isoquinoline, dansyl, quinoline or napthalene ring, conjugated to a peptide that mimics a PKB substrate.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.